In general, a shield element is mounted to an inside of a window for blocking a room from the outside, wherein the shield element may be opened or closed to control an intensity of illumination by external light, for example, sunlight.
Such a shield element is to be inevitably moved right and left, or up and down partially for the control of the illumination of a room. Therefore, the room is exposed to the outside via the window, causing invasion of privacy.
Therefore, there has been recently proposed a blind which is formed of a blind material having transparent surfaces and opaque surfaces extended repetitively to control the intensity of illumination of a room without exposing a window even partially.
FIG. 1 is a perspective view of a prior art blind for controlling height and brightness, and FIG. 2 is a longitudinal cross-sectional view of FIG. 1. Referring to FIG. 1 and FIG. 2, a prior art blind for controlling height and brightness 1 includes a winding rod 4 mounted between brackets 3, which are mounted at both ends of an upper supporting bar 2 oppositely. The winding rod 4 is mounted via a sprocket element 6 driven by a control line 5.
The upper supporting bar 2 is integrally formed with a mounting part 2a, to which an end, that is, a rear surface of the blind material 7 formed with transparent parts 8a and opaque parts 8b repetitively is fixed. The other end, that is, a front surface of the blind material 7 is fixed to the winding rod 4.
The winding rod 4 is formed with a mounting part 4a at an end and the blind material 7 is mounted with a mounting fixing bar 9a at the other end, so that the mounting fixing bar 9a is fitted into the mounting part 4a to fix the other end of the blind material 7 to the winding rod 4.
In the meantime, the upper supporting bar 2 has a mounting part 2a formed with a mounting groove 2b and the blind material 7 is fixed with a mounting fixing bar 9b at the one end, so that the mounting fixing bar 9b is fitted into the mounting groove 2b. 
As above, the blind material 7 is fixed to the upper supporting bar 2 at the one end and to the winding rod 4 at the other end, so that the blind material 7 is suspended in the shape of loop, that is, in the shape of “U” with an inner blind material 7a and an outer blind material 7b from the upper supporting bar 2.
The blind material 7 hanging down in the shape of loop is provided with a pre-weight element 10 at a lower part for keeping the blind material 7 tightly in a tensed state.
Therefore, as the control line 5 of the sprocket element 6 mounted at a side of the upper supporting bar 2 is pulled in the counterclockwise direction, the winding rod 4 rotates in the counterclockwise direction, so that the inner blind material 7a, which is fixed to the mounting part 4a of the winding rod 4, is wound on the winding rod 4, and the blind material 7 covering a window is gradually lifted.
When the inner blind material 7a is wound on the winding rod 4 and the lower part of the blind material 7 is gradually lifted, the pre-weight element 10 mounted on the loop part keeps the blind material 7 in a tight state for helping the blind material 7 to be wound on the winding rod 4 easily.
To the contrary, as the control line 5 of the sprocket element 6 is pulled in the clockwise direction, the winding rod 4 rotates in the clockwise direction, so that the blind material 7 wound on the winding rod 4 is gradually released to hang down.
When the blind material 7 wound on the winding rod 4 is released, the pre-weight element 10 mounted on the loop part keeps the blind material 7 in the tight state.
The prior art blind for controlling the height and brightness thereof by winding or releasing the blind material 7 on or from the winding rod 4 as described above has, however, a problem that an outer diameter of the blind material 7 wound on the winding rod 4 becomes increased when a thick blind material 7 is employed for a window of a predetermined height for the purpose of insulation in winter time, even though a thin blind material has no problems for a window of the same height.
The inner blind material 7a and the outer blind material 7b are kept in parallel each other without generating any serious problems if the blind material 7 is fixed to the mounting part 4a of the winding rod 4 and the mounting groove 2b of the mounting part 2a of the supper supporting bar 2 and hangs down with a maximum length, completely closing a window.
If the control line 5 is pulled out and the winding rod 4 is rotated in the winding direction, that is, the clockwise direction of the blind material 7 to expose the window, the hanging blind material 7 becomes wound on the winding rod 4 gradually and the pre-weight element 10 secured on the loop part becomes lifted. By the above operation, as the blind material 7 is wound on the winding rod 4 gradually, an outer diameter of the winding rod 4 becomes increased. Then, the inner blind material 7a and the outer blind material 7b cannot keep a predetermined distance from each other and become overlapped with each other at a wound portion of the blind material 7. Therefore, the outer blind material 7b becomes curled together with the inner blind material 7a on the winding rod 4 as the winding rod 4 rotates.
On the other hand, if the inner blind material 7a and the outer blind material 7b are kept in the parallel state with each other when the outer diameter of the blind material 7 wound on the winding rod 4 becomes maximum, then the inner blind material 7a and the outer blind material 7b are kept in the parallel state with each other when the blind material 7 is wound on the winding rod 4 to the maximum and the pre-weight element 10 is positioned at an upper dead point.
However, if the control line 5 is pulled out and the winding rod 4 is rotated in the releasing direction, that is, the counterclockwise direction of the blind material 7 to cover the window completely, a distance between the inner blind material 7a and the outer blind material 7b of the blind material 7, of which the ends are fixed to the mounting part 2a and the winding rod 4 respectively, becomes increased. Simultaneously, the blind material 7 becomes in the shape of “V” at a side view point since the pre-weight element 10 is secured on the loop part of the blind material 7. Therefore, control of a light transmission degree or blocking of light becomes impossible when making the transparent parts 8a and the opaque parts 8b accorded with each other or controlling overlapping therebetween.